Added some important information about contributing and fixed some typos

Change-Id: I48f9d8d8f092ad36ddbfc018adec2a7de33ce68e
Reviewed-on: http://review.couchbase.org/23852
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>